1. Field of Invention
The present invention relates to boost power converters ("boost converters"), and more particularly to a protection circuit of a boost converter which provides under-voltage, over-voltage, and over-current protection.
2. Background of the Invention
A boost converter is typically used to produce a higher regulated voltage from a lower unregulated voltage, including power factor correction (PFC) and DC-to-DC boost regulation. Several publications explain the operation of boost converters, such as: (a) Keith H. Billings, "Switchmode Power Supply Handbook," McGraw-Hill Book Co., p2.162-p2.166; and (b) Abraham I. Pressman, "Switching Power Supply Design," McGraw-Hill Book Co., p24-p35.
An example of a conventional boost converter 10 is shown in FIG. 1. The boost converter 10 includes a transistor Q1, inductor L1, diode D1, capacitor C1 and a PWM controller 12. The inductor L1 is connected in series with VIN and the transistor Q1. When the transistor Q1 is on for a time Ton, diode D1 is reverse biased and an energy (0.5*L1*Ion.sup.2) is stored in L1, where Ion=VIN*Ton/L1. During the transistor Q1 off time, the stored energy of L1 feeds the capacitor C1 through diode D1. Thus, controlling the Ton in the PWM controller 12 regulates the output voltage Vo.
Most power supply specifications require protection against the following common occurrences: (1) shorts to ground or overload currents, which can destroy the switching element and series-pass element; (2) output over-voltage, which can destroy voltage-sensitive loads; and (3) input under-voltage, which can not deliver sufficient power to the output and potentially will overheat the switching element. For the boost converter 10 shown in FIG. 1, when the input voltage VIN is higher than the specified output voltage Vo, the PWM controller 12 and the transistor Q1 will stop the boost switching due to feedback, but this high input voltage may unrestrainedly go into the output. Further, if the output of the boost converter 10 is shorted to ground, an unlimited current might flow from input to the output through the diode D1.
To address these concerns, protection switches, such as the conventional protection switch shown in FIG. 2, have been implemented. In the configuration shown in FIG. 2, a MOSFET Qp serves as a protection switch. The drain of the MOSFET Qp is connected to the positive output of the boost converter 10. The gate of the MOSFET Qp is connected to a gate driver 14 for driving the MOSFET Qp on, and the source of the MOSFET Qp is coupled to the load through a current sense resistor Rs. The output current IO flowing through the resistor Rs will produce a voltage drop Vrs. The resistors Ra, Rb, Rc and Rd form a voltage divider network for the voltage drop Vrs for providing an over-current signal to a control circuit 16. When the input voltage is higher than a specific level and/or the output is shorted to ground, the control circuit 16 will shut off the MOSFET Qp through the gate driver 14 to protect the boost converter 10 and the load.
Since a typical N-channel MOSFET produces lower loss, as compared to a P-channel device, the MOSFET Qp is generally an N-channel device. However, the drawback of using an N-channel device is that a specific gate driver must be applied to ensure that the MOSFET is fully turned-on. To turn-on the MOSFET, the gate-to-source voltage Vgs must be higher than a threshold voltage. If a lower on-state resistance (drain-to-source), Rds-on, of the MOSFET is needed, more Vgs voltage should be applied to the MOSFET. Although many methods can be used to drive the MOSFET, such as level-shift, charge-pump and floating source, the utilization of such methods increases the complexity of the circuit.